1. Field of the Invention
The invention relates to an anchor with a fluke and a shank, which shank can be rigid or composed of threads, and connected to an anchor line at the upper end.
2. Description of the Related Art
Such anchors are used for mooring floating objects with respect to a water bottom, such as semi-submersibles used in the exploration and exploitation of sea-bottoms.
At installation, the anchors are lowered on the water bottom and then by exerting a pulling force on the anchor line which is connected to the upper end of the shank, pulled in the ground until the anchor is sufficiently far penetrated in there for supplying the required holding power. Said anchor line, up till then used as installation line, can, if so desired, be used for connecting the object and the anchor.
For certain anchoring systems, such as so-called vertical anchoring systems, it is desirable that the mooring or load lines exert a force on the fluke which is perpendicular to the fluke as much as possible and oriented through the surface centre of gravity of the fluke. This can be realized by moving the point of engagement of the installation line on the shank to a place further rearwards on the shank, or by swinging the shank in relation to the fluke. Alternatively an extra load line can be made use of, which line is attached to the anchor on the desired place beforehand, for instance on the fluke at the location of its surface centre of gravity. Examples of such solutions have been described in applicant's international patent applications WO 93/03958 published on Mar. 4, 1993 and WO 94/12386 published on Jun. 9, 1994, the contents of which should be considered inserted herein. International patent applications WO 93/11028 published on Mar. 10, 1993 and WO 96/39324 published on Dec. 12, 1996 can also be referred to, from which anchors are known of which the angle between the shank and the fluke can be altered. In one embodiment this is realized by having the shank consist of two parts, one part extending obliquely to the fore being connected to the installation line and the other, upright part being connected to a (vertical) load line. By pulling the load line a pin breaks resulting in an uncoupling mechanism for the connection between the oblique shank part and the fluke being released. In another embodiment there is a shank, which, with the help of a removable wedge which is clamped between the shank and the fluke, is initially secured in an oblique position. By pulling an extra pulling line the bolt breaks after which a bar provided with a wedge at its bottom end can be slid upwards along the shank in order to lift the wedge, after which the shank can be turned upright. In yet another embodiment the angle is enlarged by swing-pulling the shank from the installation position to a vertical position with the help of the anchor line, by swing-pulling the shank and having a connection between the shank and the fluke fail therewith.
It can also be desired to retrieve the installation line after having pulled an anchor into the ground, possibly together with the shank. For connecting (the rest) of the anchor with the object, an extra anchor line has then already been attached to the fluke or with the shank (when it remains connected to the fluke). The connection between the installation line and the shank or either the connection between the shank and the fluke can be adapted to that end in order to fail at a certain pulling force. Alternatively the connection shank-fluke can be remotely operable for uncoupling, for instance with an extra pulling line. Examples of anchors which have been adapted to that end have been described in the aforementioned international patent application PCT/NL92/00144.
It can furthermore be desirable to alter the angle between the shank and the fluke in order to be able to pull the anchor, at least the fluke, out of the ground to be able to use the (valuable) anchor again. From the international patent application PCT/NL92/00144 an anchor is known of which the shank is connected to the fluke on two locations spaced apart in longitudinal direction of the fluke, one of the connections being remotely detachable, for instance with an extra pulling line or in an acoustic manner, and the other, preferably foremost connection is a hinge. By releasing the connection mentioned first the fluke will only be connected to the fluke at the location of the hinge connection, wherein the fluke can direct itself in an orientation of the lowest resistance when pulling out the anchor. In one embodiment the connection mentioned first is also adjustable, because of which the angle between the shank and the fluke can be enlarged in order to be able to deploy the anchor in the aforementioned vertical anchoring systems.
For all these conversions it is necessary to have a part fail before the connection concerned can be uncoupled and/or special aids, such as pulling lines, acoustic means and hydraulic means are necessary. In this way, on the one hand, there is the risk that the connection concerned releases at an unforseen exceeding of the failure limit of the part (long) before such is desired and the anchor has to be hauled in again in order to install it again or that a future possibility of conversion has to be given up. If one would want to avoid this risk--if at all possible--one would have to manoeuvre the anchor very carefully.
On the other hand the aids mentioned make the anchor expensive and prone to damage and therefore sensitive to disturbance regarding the conversion function.
An objective of the invention is to provide an anchor in which in a simple and reliable way, at the desired moment, one of the types of conversions mentioned, from the installation stage to the user stage or from the user stage to the hauling-in stage, can take place. Furthermore it is an objective of the invention to provide a method for this.